NPY and major depressive disorder: In addition, decreased NPY concentrations are associated with conditions such as schizophrenia, depression, bipolar disorder, and Alzheimer's disease (Beal et al., 1986b; Kowall and Beal, 1988; Caberlotto and Hurd, 1999; Kuromitsu et al., 2001; Frederickson et al., 2007; Morales-Medina et al., 2010).